The present invention relates to a backflow prevention apparatus of a clean room applied to a dust-free room or a germ-free room of a semiconductor manufacturing plant, a FPD (Flat Panel Display) manufacturing plant, a precision machine plant, a pharmaceutical manufacturing plant, or the like.
Conventionally, there is a total down-flow system as shown in FIG. 6 as a system for realizing a clean room with a high cleaning level. With this system, the air in a ceiling chamber 264 flows into a clean room 262 from air intake ports of fan filter units (hereinafter, FFUs) 217 installed in a ceiling of the clean room 262, pressure thereof is boosted by an internal air blower, dust is removed by a high-performance filter, and then the clean air flows in the vertically downward direction in the clean room 262. Next, a circulation flow is formed in such a manner that the air passes through a grating floor 261 of the clean room 262, flows into an underfloor chamber 263, and returns to the ceiling chamber 264 via a return flow path 266. By such a circulation, the dust of the same air is removed several times by the high-performance filter. Thus, after a certain fixed time elapses after operation of the clean room 262 is started, a high cleaning level is maintained in the clean room 262.
In a semiconductor manufacturing plant or a FPD manufacturing plant, in accordance with high integration of devices, environmental conditions such as a cleaning level, a temperature, or humidity are required to be controlled at higher level. Further, due to tougher price competition of a semiconductor or a FPD in recent years, construction cost of the clean room, that is, initial cost, and running cost of the clean room itself are required to be reduced. Thus, there are attempts to reduce the installment number of clean air blowoff devices such the FFUs.
In the clean room 262, in general, a number of manufacturing devices 265 provided only with FFUs or fans are installed, and the air taken in by the manufacturing devices is often exhausted to the external air or directly exhausted to the underfloor chamber 263, so that an air amount in the clean room 262 is decreased. Therefore, in a case where the installment number of the clean air blowoff devices such as the FFUs 217 installed in the ceiling 264 of the clean room is reduced, a point where pressure in the clean room 262 is lower than pressure of the underfloor chamber 263 is generated, so that a backflow from the underfloor chamber 263 into the clean room 262 is generated.
When the backflow from the underfloor chamber 263 into the clean room 262 is generated, an air flow in the vertically downward direction in the clean room 262 is largely disturbed so as to cause deterioration of the cleaning level. Further, in general, a pump, a chemicals tank, pipes, and the like are arranged in the underfloor chamber 263. Thus, grit and dust accumulated and attached on surfaces of these auxiliary facilities fly together with the backflow air and flow into the clean room 262, so that contamination remarkably progresses. Such a contamination problem due to the backflow is an important problem that is more unavoidable as saving facilities and energy of the clean room 262 is more facilitated to come close to a limit design.
There is a conventional technique that a sensor for detecting the flow direction or velocity of the air is arranged at a position in the vicinity of a floor, and a control means for adjusting a flow rate of the clean air blown off from the FFUs 217 installed in the ceiling of the clean room in accordance with a detected value of the sensor is provided.
This control means is a differential pressure gauge for indirectly detecting the flow direction or the velocity of the air by detecting differential pressure of upper and lower spaces taking the floor as a border by the sensor installed in the vicinity of the floor of the clean room. The control means adjusts the flow rate of the clean air blown off from the FFUs 217 installed in the ceiling 264 of the clean room in such a manner that the differential pressure detected by the differential pressure gauge is within a fixed range, so that an effect of reducing the installment number of the clean air blowoff devices such the FFUs 217 installed in the ceiling 264 of the clean room without generating the backflow from the underfloor chamber is clearly described (for example, refer to Japanese Unexamined Patent Publication No. 2004-218919).